Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms can be categorized by technology as either drop-on-demand ink jet or continuous ink jet.
The first technology, drop-on-demand ink jet printing, typically provides ink drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the print head and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image. With thermal actuators, a heater, located at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble. This increases the internal ink pressure sufficiently for an ink drop to be expelled. Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to vanLintel, on Jul. 6, 1993, have a piezoelectric crystal in an ink fluid channel that flexes in an applied electric field forcing an ink drop out of a nozzle.
The second technology, continuous ink jet printing, uses a pressurized ink source that produces a continuous stream of ink drops. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink drops. The ink drops are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink drops are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When print is desired, the ink drops are directed to strike a print medium.
U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink drops to be printed are selectively charged and deflected towards the recording medium. This early technique is known as electrostatic binary deflection continuous ink jet.
U.S. Pat. No. 4,636,808, issued to Herron et al., U.S. Pat. No. 4,620,196 issued to Hertz et al. and U.S. Pat. No. 4,613,871 disclose techniques for improving image quality in electrostatic continuous ink jet printing including printing with a variable number of drops within pixel areas on a recording medium produced by extending the length of the voltage pulses which charge drops so that many consecutive drops are charged and using non-printing or guard drops interspersed in the stream of printing drops.
Later developments for continuous flow ink jet improved both the method of drop formation and methods for drop deflection. For example, U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink drops through the use of transducers. The lengths of the filaments before they break up into ink drops are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitude stimulations resulting in longer filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air-flow affects the trajectories of the filaments before they break up into drops more than it affects the trajectories of the ink drops themselves. By controlling the lengths of the filaments, the trajectories of the ink drops can be controlled, or switched from one path to another. As such, some ink drops may be directed into a catcher while allowing other ink drops to be applied to a receiving member.
U.S. Pat. No. 6,588,888 entitled “Continuous ink-jet printing method and apparatus,” issued to Jeanmaire, et al. (Jeanmaire '888, hereinafter) and U.S. Pat. No. 6,575,566 entitled “Continuous inkjet printhead with selectable printing volumes of ink,” issued to Jeanmaire, et al. (Jeanmaire '566 hereinafter) disclose continuous ink jet printing apparatus including a droplet forming mechanism operable in a first state to form droplets having a first volume traveling along a path and in a second state to form droplets having a plurality of other volumes, larger than the first, traveling along the same path. A droplet deflector system applies force to the droplets traveling along the path. The force is applied in a direction such that the droplets having the first volume diverge from the path while the larger droplets having the plurality of other volumes remain traveling substantially along the path or diverge slightly and begin traveling along a gutter path to be collected before reaching a print medium. The droplets having the first volume, print drops, are allowed to strike a receiving print medium whereas the larger droplets having the plurality of other volumes are “non-print” drops and are recycled or disposed of through an ink removal channel formed in the gutter or drop catcher.
In preferred embodiments, the means for variable drop deflection comprises air or other gas flow. The gas flow affects the trajectories of small drops more than it affects the trajectories of large drops. Generally, such type of printing apparatus that causes drops of different sizes to follow different trajectories, can be operated in at least one of two modes, a small drop print mode, as disclosed in Jeanmaire '888 or Jeanmaire '566, and a large drop print mode, as disclosed also in Jeanmaire '566 or in U.S. Pat. No. 6,554,410 entitled “Printhead having gas flow ink droplet separation and method of diverging ink droplets,” issued to Jeanmaire, et al. (Jeanmaire '410 hereinafter) depending on whether the large or small drops are the printed drops. The present invention described herein below are methods for implementing small drop printing modes.
Jeanmaire '888 and Jeanmaire '566 disclose the concept of continuous inkjet printing wherein the smallest volume drops are used for forming the image pattern on a receiver medium and large drops are formed and guttered to capture excess jetted liquid or liquid that would otherwise strike the media in non-print areas. However, Jeanmaire '888 and Jeanmaire '566 do not disclose methods for translating input image or pattern data into jet stimulation pulse sequences that break up a jet into sequences of print and non-print drops that will result in an acceptable liquid pattern image at the receiver medium. Implementation of a small drop print mode requires that the sequences of jet break up pulses applied to each jet of a plurality of jets be formed based on the desired optical density or liquid deposition amount at each output image picture element (pixel) as well as the characteristics that the large non-print drops must be given for reliable deflection path discrimination and capture by the gutter.
Further, small drop printing offers a better opportunity to provide more levels of gray scale at each pattern pixel location and to alter the position and shape of the printed ink within a pixel area. However, to take advantage of the print quality opportunities offered by small drop printing, practical and efficient methods of translating input image and pattern data into useful drop forming pulse sequences are needed.